A molded product having an intended cross sectional shape has been conventionally obtained by the known method that a sheet-like composite material, namely, a prepreg manufactured by impregnating a reinforcing material like a carbon fiber, an aramid fiber or a glass fiber with a thermo-setting resin called matrix like an epoxy resin or a phenol resin, is heated and pressure-formed.
These fibers are formed into plate-like fiber layers. A plurality of the fiber layers are laminated to form the composite material in a manner that the fibers of the layers are arranged in different directions. By doing so, a lightweight and strong product can be obtained since the carbon fiber, the glass fiber etc. have high coefficient of elasticity, and the product is widely used in aircraft, automobile and other general industries.
The composite material including the thermosetting resin as the matrix has the behavior that it is soft at ambient temperatures and is cured reactively by heating to a predetermined temperature.
One of the technologies for molding the composite material is the technology in which a hot-press apparatus is used. According to this technology, the composite material is placed between an upper metal mold and a lower metal mold in a sandwiching manner as shown in FIG. 14. The composite material is then heated and pressurized based on a pattern graph of FIG. 15 for controlling the temperature and pressure over the time course. When the resin is finished curing, a molded product having a predetermined cross sectional shape is obtained as shown in FIG. 14. The metal molds generally have electric heaters or dedicated coils therein to heat a material. The metal molds are heated by heat conduction or electromagnetic induction, and the heat is used for molding.
The composite material is formed of a carbon fiber, an aramid fiber or the like, and the resin called matrix as mentioned above. Taking an epoxy resin for instance, when it is heated to around 90-100° C. at which viscosity of the resin becomes the lowest, the resin having viscoelasticity at an ambient temperature reaches a softening point and the fluidity is increased. By maintaining the temperature, the air contained in the material and the air confined between the laminated layers go out and a hollow called a void is not left in the product. This step is called a dwell step.
After this step is finished, the composite material is continuously heated to a predetermined temperature, and at the same time, pressurization is started to obtain a predetermined pressure. The pressure is increased so that it may reach a predetermined pressure a little before the predetermined temperature is obtained, and the predetermined temperature and pressure are maintained. Curing starts at the predetermined temperature, and the temperature is maintained until curing is completed. Normally, the curing is completed in about an hour. The speed of temperature rising varies depending on a total thickness of the laminated material. To be specific, the thicker the material is, the slower the temperature rises. When the temperature rises quickly upon heating the material, variation in temperature is caused, and therefore variation in curing state is caused. Furthermore, the variation in curing state may cause fracture due to lack of strength. The speed of temperature rise is determined based on experience, experiments, etc. As for the speed of pressure rise, it is only necessary to raise the pressure so that the pressure curve reaches the predetermined pressure a little before the temperature reaches the predetermined temperature.
The above-mentioned pressure profile is only an example in which the pressure starts rising after the dwell step. However, pressure may be raised at the same time as the dwell step or before the step. Further, the dwell step itself may be omitted.
A molded product of a composite material of this kind can also be obtained by an autoclave molding method in addition to the above-mentioned method using the hot-press apparatus.
According to the autoclave molding method, the composite material is placed in a molding chamber. Then, pressurized air, nitrogen, or mixed gas of these is supplied, and the composite material is heated to a predetermined temperature by a heating means and is pressurized for molding.
In this occasion, the heated air is circulated in the molding chamber so that the heat is evenly applied to the composite material.
The following documents are listed as conventional technologies related to the above-mentioned hot-press apparatus and the autoclave molding method.
(Document 1) Unexamined Patent Publication No. 2010-115822
(Document 2) Unexamined Patent Publication No. 2006-88049
(Document 3) Unexamined Patent Publication No. 2009-51074
According to the molding method using the above-mentioned hot-press apparatus, since the composite material is molded by surface pressure applied in one direction by the upper metal mold and the lower metal mold, no pressure is applied to the faces perpendicular to the surface pressure. Consequently, density of the composite material in the direction perpendicular to the direction of the surface pressure is insufficient and an inherent strength can not be obtained, which constitutes a problem. In a product having an uneven and complicated cross section, the problem is more conspicuous. When a composite material molded product including different cross sections is to be formed by using the hot-press apparatus, unified molding is difficult and therefore a plurality of members have to be bonded together, making the process complicated. Further, in the case of the hot-press apparatus, since electric heaters and dedicated coils have to be arranged in the upper and lower metal molds, the metal molds themselves become expensive. Furthermore, since the metal molds are required to have fatigue strength and robustness against pressure, they are easy to become large and heavy, which also constitutes a problem in the cost of the metal molds.
On the other hand, an autoclave molding method is suitable for molding a product having a complicated cross sectional shape. However, there are various problems in heating and pressurizing.
Heated air or nitrogen is generally used to supply heat to the composite material. The composite material is placed in a vacuum bag and then placed in a molding chamber, and heat needs to be applied evenly to the composite material by heated air or other gases. For that purpose, it is necessary to arrange a circulation means like an electric motor and a fan for circulating the heated air, and a heating means like a heater for continuous supply of heat. This makes the apparatus large and requires control of the apparatus.
The air, nitrogen or mixed gas of these used in the autoclave molding method like the abovementioned has less heat conductivity compared to that of a metal like the metal mold. Therefore, the gas serves as a heat insulating material. In addition, the temperature rising ratio of the composite material is small partly because the gas has lower heat capacity per unit flow. Further, it is necessary to circulate the gas with preferable flow to allow the heated gas to come into contact with the composite material evenly, which has been difficult.